The present invention is generally directed to fuel cell components, and to solid oxide fuel cell electrolyte materials in particular.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. Electrolyzer cells are electrochemical devices which can use electrical energy to reduce a given material, such as water, to generate a fuel, such as hydrogen. The fuel and electrolyzer cells may comprise reversible cells which operate in both fuel cell and electrolysis mode.
In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 650° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit. A solid oxide reversible fuel cell (SORFC) system generates electrical energy and reactant product (i.e., oxidized fuel) from fuel and oxidizer in a fuel cell or discharge mode and generates the fuel and oxidant using electrical energy in an electrolysis or charge mode.
Scandia stabilized zirconia (SSZ) SOFC electrolyte material exhibits a high oxygen ion conductivity. Typically, zirconia is doped with between 8 and 11 mol % scandia (Sc2O3) in order to stabilize the cubic phase zirconia at high SOFC operating temperature of 650-850° C.
However, there are two problems with SSZ electrolyte materials: 1) they exhibit a cubic to rhombohedral phase transformation at around 580° C., and 2) the ionic conductivity slowly decreases with time, which is known as ageing.
Others have shown that co-doping SSZ with one secondary rare earth oxide will suppress the cubic to rhombohedral phase transformation. For example, 10Sc1Ce zirconia (10 mol % Sc2O3—1 mol % CeO2—zirconia) and 10Sc1Y (1 mol % Sc2O3—1 mol % Y2O3—zirconia) are examples of co-doped zirconia compositions that do not exhibit the cubic to rhombohedral phase transformation.
However, both of these zirconia compositions still experience ageing (i.e., the a slow decrease in ionic conductivity with time at the SOFC operating temperatures of 800-850C). Although the actual mechanism of this ageing degradation in ionic conductivity with time is not agreed upon in the scientific literature, one hypothesis is that the cubic phase is not stable and slowly decomposes into a tetragonal phase which has a lower ionic conductivity. The resulting material consists of a two-phase mixture primarily of cubic phase with small domains (e.g., 2-5 nm) of tetragonal phase. As shown in FIGS. 1 and 2, the 10Sc1Ce zirconia may exhibit a faint superlattice of tetragonal phase as well as the cubic fluorite structure of zirconia.